Encroachment of aqueous solutions distinct in ionic character from connate water into subterranean sandstone formations containing clays often results in reduced fluid flow, and therefore, reduced oil production. Encroachment of aqueous solutions having a distinct ionic makeup in a subterranean sandstone formation can occur in a variety of ways, such as, invasion by an underlying aquifer, invasion of a secondary or tertiary oil recovery flood, and invasion of certain fluids utilized in the well bore or in the near well bore environment. In particular, fluids positioned within a well bore during workover operations, or fluids utilized to kill a well prior to a subsequent operation, or fluids utilized to complete a well bore for subsequent production or injection of fluids, tend to inadvertently invade the near well bore environment of a subterranean sandstone formation and may result in formation permeability damage and reduced fluid flow.
Two distinct types of clay damage can result from encroachment of aqueous solutions having a distinct ionic makeup. First, swellable clays, such as montmorillonite, have interstitial layers. Fresh water contact affects the ionic properties between these interstitial layers and swells these clays thereby impeding fluid flow therethrough. Secondly, migratable clays, such as poorly cemented kaolinite and illite clay particles, become detached from the subterranean sandstone formation during flow of fresh water therethrough. The resultant mobile clay particles can become trapped in the formation porethroat openings, and thus, reduce permeability and fluid flow therethrough. The second type of permeability reduction is referred to as clay particle migration. Often, encroachment of an aqueous solution having a distinct ionic makeup, for example, fresh water, into a subterranean sandstone formation containing clays results in the occurrence of both types of permeability damage.
Several prior art processes have been proposed to stabilize clays present in subterranean formations, and therefore, alleviate fresh water damage thereto. U.S. Pat. No. 3,640,343 to Darley discloses a method for stabilizing hard shaly earth formations (i.e., migratable clays) during drilling or fluid production by injecting into the hard shaly earth formation a dilute aqueous solution of alkali metal silicate containing SiO.sub.2 in an amount from about 2 to about 6 percent by weight and having a viscosity of less than 2 centipoise and a pH of from about 11 to about 12. It has been proposed to inject an aqueous solution of potassium chloride or a soluble polyvalent cation salt solution, such as, calcium or magnesium salt solution, into a subterranean sandstone formation to stabilize clays. However, potassium chloride and polyvalent cation salt solutions will stabilize clays only when the connate brine in contact with the clay has a high potassium-to-sodium ion ratio or a relatively low polyvalent cation concentration. Clay stabilization resulting from treatment with potassium chloride solutions has proven only temporary in that most formation and injection waters have high sodium-to-potassium ion ratios, and as such, potassium ions are rapidly exchanged from the clays resulting in the loss of any clay stabilization attributable to the potassium ions. Clay stabilization resulting from ordinary polyvalent cations, such as, calcium and magnesium, affords only temporary protection against fresh water because such polyvalent cations are rapidly exchanged from the clays. Other proposed clay stabilization treatment fluids include a solution containing water-soluble organic polymers, a hydroxyaluminum, acidic solution, such as set forth in U.S. Pat. No. 3,603,399 to Reed, a calcium hydroxide solution, such as set forth in U.S. Pat. No. 4,031,959 to Henderson, a fluroboric aqueous acidic solution, and a soluble zirconium salt solution. Utilizing sodium hydroxide to stabilize clays has proven relatively ineffective in that sodium hydroxide can promote significant formation permeability damage, and in some instances, actually increases the fresh water sensitivity of formation clays.
Sodium hydroxide is often employed in well bore completion fluids for conditioning a well bore for cementing. Such completion fluids tend to inadvertently invade the near well bore environment of a water-sensitive, subterranean sandstone formation. Sodium hydroxide does not impede permeability damage due to such encroachment, but often actually reacts with certain silicate minerals present in formation to form plugging precipitates and gels which further reduce permeability.
The above treatment solutions, most of which do not contain hydroxide ions, have resulted in varying effectiveness, are relatively expensive to utilize or result in adverse in situ side effects, such as permeability reduction. Thus, a need exists for processes for working over, killing and completing a well bore penetrating a subterranean sandstone formation which do not result in substantial fresh water permeability damage to a subterranean formation containing clays and which stabilize such clays for a substantial period of time.